A frequent goal in designing wireless communications systems is to increase the number of users that may be simultaneously served by the communications system. This may be referred to as increasing system capacity.
In code division multiple access (CDMA) wireless communications systems, the use of adaptive antenna arrays at the base transceiver has been proposed as a method of increasing system capacity. An adaptive array antenna includes two or more radiating elements with dimensions, spacing, orientation, and an illumination sequences such that the fields for the individual elements combine to produce a field having greater intensities in some directions and lesser field intensities in other directions. An adaptive array antenna helps increase system capacity because this field pattern or radiation pattern (which includes a plurality of beams or lobes) may be configured such that signals intended for a selected user are in higher-gain antenna lobes are pointed in the direction of a propagation path to a selected user, while nulls in the antenna pattern are likely to be directed to other users. Thus, other signals intended for the other users in the selected user's antenna null are not adversely affected by the power intended for the selected user. This increases capacity because one user's signals are not transmitted with a higher antenna gain to all the other users in the sector or cell where it would degrade all other users' signals. While some other users may be in a higher gain lobe, others are not, which makes all users statistically better able to receive their intended signals.
In prior art proposals for adaptive array transmitters that adjust their patterns on a per user basis, a per user pilot is typically used. This is because proper demodulation requires that the pilot be in phase with the traffic channel. Thus, if the pilot is not transmitted with the same antenna pattern as the traffic channel, then the pilot phase will be shifted relative to the traffic channel. In an adaptive array system having a pilot for each user, each user's pilot must be modified in accordance with the weights (i.e., the gains and phases) used for creating the user's traffic channel illumination sequence.
This the per user pilot system has diminished capacity due to: (1) the use of additional pilots; (2) the use of longer pilot sequences to distinguish each pilot; (3) a corresponding increase in complexity of the pilot searcher in the subscriber unit due to the longer pilot sequences; (4) the lack of backward compatibility with existing, CDMA cellular (IS-95) subscriber units; and (5) the increase in soft handoff complexity, and reduction in capacity due to the assignment of an additional pilot per user during soft handoff.
Adding a per-user pilot effectively reduces the amount of gain obtained by using an adaptive array. If we assume that a broadcast pilot takes up 7% of the total transmit power, and that per-user pilots use the same 7% of total transmitted power, 7% capacity is lost because broadcast pilots are still required for handoff purposes.
Because of the increase in sector capacity provided by adaptive arrays, four times as many pilots may be required. This means that the pilot sequences must be four times longer in order to distinguish the pilots. A searcher in a subscriber that searches for these longer pilots would require four times as much integration time, which means higher computational requirements. This is compounded by the fact that the narrow beams of the adaptive arrays generally mean more searching has to be done since there are more beams. This means that the searchers are likely to be overloaded.
Thus, it should be apparent that a need exist for a method and system of transmitting and demodulating a communications signal with an adaptive antenna array without the need for per-user pilots or high-capacity, complex signaling between the base transceiver and the subscriber unit.